Active elements modified chromium diffusion patch coating

ABSTRACT

There is provided a method for applying a diffusion coating on a specific area of targeted industrial item such as a turbine blade. The method uses a covering material such as a tape or slurry to cover the area where it is desired that the diffusion occur, for example above the root area of a turbine blade. The tape material includes a metallic source such as chromium and a master alloy of active elements for diffusion. The covering material thus defines the localized patch that is to be coated. An activator if any, such as a halide activator, can be included in the tape or slurry. Alternatively, the activator can be included in the pack material. The method uses known pack cementation methods to complete the diffusive process. The method results in a diffusion coating over a specific area of the target item.

FIELD OF THE INVENTION

The present invention relates to methods and materials for forming aprotective diffusion layer on metallic items. More particularly theinvention relates to a method for applying an active elements modifiedchromium diffusion layer coating on a specific area of an item, such asbetween the serrations and platform on a turbine blade, withoutdiffusing coating material into other areas of the item.

BACKGROUND OF THE INVENTION

In an attempt to increase the efficiencies and performance ofcontemporary jet engines, and gas turbine engines generally, engineershave progressively pushed the engine environment to more extremeoperating conditions. The harsh operating conditions of high temperatureand pressure that are now frequently specified place increased demandson engine components and materials. Indeed the gradual change in enginedesign has come about in part due to the increased strength anddurability of new materials that can withstand the operating conditionspresent in the modern gas turbine engine.

The turbine blade is one engine component that directly experiencessevere engine conditions. Turbine blades are thus designed andmanufactured to perform under repeated cycles of high stress and hightemperature, as well as under corrosive atmospheres. An economicconsequence of such a design criteria is that currently used turbineblades can be quite expensive. It is thus highly desirable to maintainturbine blades in service for as long as possible. It is correspondinglydesirable to manufacture and finish turbine blades so as to withstandthe corrosive and erosive forces that will attack turbine bladematerials.

Turbine blades used in modern jet engines are frequently castings madefrom a class of materials known as superalloys. The superalloys includealloys with high levels of nickel and/or cobalt. In the cast form,turbine blades made from superalloys include many desirable physicalproperties such as high strength. Advantageously, the strength displayedby this material remains present even under stressful conditions, suchas high temperature and high pressure. Disadvantageously, thesuperalloys generally can be subject to corrosion and oxidation at thehigh temperature operating regime. Sulfidation can also occur in thoseturbine blades subject to hot exhaust gases.

Thus, it has become known to provide coatings or protective layers onitems, such as turbine blades, that are subject to corrosion, erosion orsulfidation. Chromium, aluminum, and other metallic coatings can be usedto provide a protective layer that is more resistant to corrosion and/oroxidation than is the underlying substrate material. In the case gasturbine engine components made out of superalloys, materials such asplatinum, aluminum, and chromium can be used to provide protectivediffusion coatings.

One method used for providing diffusion coatings is the pack cementationprocess. In this method the target, the industrial item to be coated, isplaced in a box or retort with a “pack” surrounding it. The packtypically includes a source of the metal that is to be diffused into thetarget, inert packing material, and an activator. Typically the targetlies in a bed of powdered materials. The box containing the target andits surrounding pack is then placed in an oven where the materials areheated for desired periods of time at desired temperature ranges.Diffusion takes place during the heat treating thermal cycle process.Pack cementation is a comparatively attractive method of coating in thatit is a relatively simple method that is relatively inexpensive to applyto the target, as compared to other methods of coating superalloys.

The damage caused by oxidation, hot corrosion, and erosion, due to theflowing of hot combustion gases often establishes the operating lives ofjet engine components such as blades and vanes. Hence the components arecoated with various types of coatings to meet and extend the operatinglives of parts. Thus the high pressure turbine (HPT) blades and vanesgenerally utilize aluminides and platinum modified aluminides foroxidation resistance. In the low pressure stages, chromium diffusioncoatings are used more advantageously. Also, for components such as HPTblades, the tip areas require oxidation protection whereas the shankareas between an airfoil platform and root serrations often requirebetter high temperature sulfidation (corrosion) resistance. Therefore,in order to tailor the coating requirements for OEM and repairapplications, there is a necessity to provide localized or patch coatingprocess capability.

In the coating industry there are available several localized typealuminide coating processes. The “codal” coating (General ElectricCorp.), “PWA 545” (United Technologies), and “Sermalloy J” (SermatechInternational Inc.) are examples for applying a localized diffusedaluminide coating. Such localized/patch aluminide coatings have been andare continued to be used in the coating industry. However, the art ofapplying a localized chromium diffusion coating has not taught the useof an active elements (hafnium, silicon, yttrium, etc.) modifiedchromium patch process. Therefore, a need exists for developing advancedactive elements modified chromium diffusion coatings for improvedperformance characteristics.

The patch coating process using a tape or slurry application essentiallyfollows the chemical vapor deposition procedure that occurs in the packcementation process. Hence, coating formation with simultaneouscodeposition of multiple elements can become somewhat complex.Codeposition of dual elements and some processing considerations aredescribed in U.S. Pat. Nos. 5,364,659; 5,589,220; 3,779,719; 5,972,429;and 6,387,194. In addition to providing a general background, thereferenced prior art teaching has brought forth the limitations incontrolling the thermodynamic activities and vapor pressures forco-deposition of two or more elements in the development of localizedpatch coating. The situation furthermore becomes increasingly intricatewith the need for diffusion of more than two elements for developmentand formation of active elements modified chromium patch coatings.

The prior art methods of providing protective coatings have neverthelessexperienced additional limitations and drawbacks. One problem that hasbeen encountered is the inability of known methods to apply activeelements modified chromium coatings only on selected areas of the targetitem rather than on the entire surface area of the item. For example, inturbine blades there is a desire to coat areas “above the platform” thatis, the airfoil, with active elements modified chromium (onwardsreferred to as AEMC) while leaving other areas, the serrations, withouta coating. Yet, in other cases, there is a desire to coat areas betweenserrations and blade platform (also sometimes referred to as the shankportion) with AEMC, while the blade airfoil and platform receive adifferent functional coating. However, traditional pack cementationmethods cannot direct the diffusion to only one area of the turbineblade.

For instance, it is desired to apply a localized AEMC diffusion coatingon an item such as a turbine blade when, for example, the airfoil isreceiving other type coatings not intended or otherwise well applied tothe shank area. For example, in some applications an MCrAlY type coatingis applied to an airfoil and platform region of a turbine blade throughan Electron Beam Physical Vapor Deposition (EBPVD) process. Such aprocess, however, depends for its effectiveness on a line of sightapplication of the MCrAlY coating. In the shank area of the turbineblade, the process can thus suffer from poor or now coatingapplicability. Thus some turbine blades receive an MCrAlY coating on theairfoil and do not receive a protective coating on the shank area.

Further, areas “below the platform” and above the serrations or root ofa turbine blade may also be subject to different forms of environmentaldegradation compared to the airfoil region of the blade. In someinstances the shank area of a turbine blade requires primarilysulfidation resistance to Type I and Type II hot corrosion conditions.Chromium and AEMC is an appropriate type coating to protect against thiskind of attack. Thus, it would be desired to provide one kind of coatingon a turbine blade airfoil (such as high temperature oxidationresistance) while providing an AEMC diffusion coating below theplatform. A coating method is therefore needed that can be applied toone area of a turbine blade while not interfering with the coatingrequirements at a different area of the turbine blade.

Other instances where a localized diffusion coating would be desiredinclude the seal slot locations in shrouds and ducts as well as sealslots in vane components.

Hence there is a need for an improved method to apply an active elementsmodified chromium diffusion protective coating on a metallic item suchas a turbine blade. There is a need for an improved coating method thatcan limit the diffusion of all desired elements to a specific area whileavoiding the diffusion in other areas. Moreover there is a need for animproved diffusion method that retains the cost advantages associatedwith known diffusion methods. The present invention addresses one ormore of these needs.

SUMMARY OF THE INVENTION

The present invention provides a method and materials for applying adiffusion coating on a specific area of a targeted industrial item suchas a turbine blade. The method uses a covering material such as a tape,putty, or slurry to cover the area where it is desired that thediffusion occur. The tape material includes metallic sources such aschromium and active elements containing master alloy and/or activeelements in metal form for diffusion. The covering material thus definesthe localized patch that is to be coated. An activator if any, such as ahalide activator, can be included in the tape or slurry. Multipleactivators can also be used in the tape/slurry or pack make up.Alternatively, the activator can be included in the pack material. Themethod uses known pack cementation methods to complete the diffusiveprocess.

In one embodiment, and by way of example only, there is provided amethod for diffusion coating a specific, localized surface of a metallictarget comprising the steps of: applying a tape to a surface of a targetwherein the tape is impregnated with metal powder comprising chromiumand elements selected from the group consisting of silicon, hafnium,tantalum, rhenium, and yttrium and/or master alloy powder containingactive elements; and wherein the tape further comprises a bindermaterial; placing a halide activator (or multiple activators) inproximity to the surface of the target; and heating the target to atemperature sufficient to cause diffusion of the desired metals in thetape into the surface of the target. The step of applying a tape mayfurther include applying a tape wherein the tape is impregnated with asingle or multiple halide activators. The step of applying a tape mayalso include a tape impregnated with metal powder comprising a chromiumalloy containing elements selected from the group consisting of silicon,hafnium, tantalum, rhenium, and yttrium. The tape is shaped to cover adesired area of the target. The tape may also include an inert filler.The step of heating the target may further include heating the tapecovered target to a temperature between approximately 1800° F. and2100°60 F. and holding the temperature therebetween for between about 2to about 16 hours. The activators may include encapsulated halideactivators as well.

In a further embodiment, also by way of example only, there is provideda turbine blade ready for diffusion comprising: a turbine blade defininga surface; a tape affixed to a surface of the turbine blade wherein thetape comprises a binder, an inert filler, a halide activator, a chromiumcontaining powder, and a powder of master alloy and/or elementscontaining metals selected from the group consisting of: silicon,hafnium, tantalum, rhenium, and yttrium. The tape may include anadhesive, and the tape may be affixed to the airfoil of the turbineblade.

In further embodiments the activators, which may include halideactivators, are either encapsulated or non-encapsulated, and may bepresent in either slurries, putties, tapes, and/or in the diffusionpacking.

Other independent features and advantages of the method of patchdiffusion coating will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the pack cementation method according toan embodiment of the present invention.

FIG. 2 is a representative view of a Mar M 247 test bar showing patchchromium diffusion at the bright contrast area where tape was affixed.

FIG. 3 is a view of patch chromium diffusion on a convex airfoilsurface.

FIG. 4 is an SEM micrograph depicting chromium diffusion coating on MarM 247 test bars.

FIG. 5 is an SEM analyses of chromium diffusion coating with threelayers of a T-1555 tape application.

FIG. 6 is an SEM analyses of chromium diffusion coating with one layerof T-1558 tape application.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.Reference will now be made in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

It has now been discovered that a localized active elements modifiedchromium (AEMC) diffusion coating can be applied to a specific area ofan industrial item. In the example of a turbine blade, an AEMC coatingcan be applied to the shank area below the platform of the blade whilethe airfoil of the turbine blade does not receive that form of coating.The method takes advantage of tapes, putties, or slurries that can befabricated to cover or preplace in the desired areas. Chromium, andother desirable metals, are incorporated in the tapes, slurries, orputties. The areas covered by the tape, slurry, or putty, receive thediffusion of the desired metals during the thermal cycles employed forcoating formation.

In one preferred embodiment a tape, slurry or putty is prepared usingchromium or chromium master alloy powder containing active elementsand/or one or more active elements, a carrier, and a binder. Preferablyall metal alloy powders used have a mesh size equal to or below 140mesh. In this embodiment an activator and/or multiple activators, ifany, is not included in the slurry, tape, or putty. Rather activatorsare placed in the pack.

In an alternative embodiment, an activator (or activators) is includedin the tape, slurry, or putty. For an AEMC coating, a halide compound isa preferred activator. Activators can be used also in the patchformulations and pack make-ups in combination to accomplish synergisticeffects.

The coating tapes that may be used according to one embodiment of thepresent invention comprise a metal source for all elements (Cr, Hf, Si,Y, etc.), a filler material, a binder material, and optionally, thoughpreferably, an activator material.

The metals in the tape include chromium and at least two or more activeelements. The chromium source may be elemental chromium or chromiumalloy. Preferably a high purity chromium powder of 99.9% or greaterpurity is used. Active elements may include silicon, hafnium, yttrium,tantalum, and rhenium. Again these elements may be present in elementalform, or in master alloy form.

Some preferred compositions of the active elements containing masteralloys area as follows, with weight percentages being nominal. NominalComposition of Master Alloy A B C D E Hf 25% 30% 40% 30% 40% Ni  5% 10%15% 15% 20% Y 0.5%  0.5%  0.5%  5.0%  10% Si bal. bal. bal. bal. bal.

A further embodiment adds additional materials such as zirconium,rhenium, and tantalum. These elements can be added up to 50% by weightin A, B, C, D, and E master alloy formulations.

It is within the scope of the invention to provide metal powder that iseither elemental of each metal or is an alloy of metals. Further thecombination of metals in elemental form with metals in alloy form can beadjusted to affect the thermodynamic activity with respect to a givenhalide activator or activators. Metals in their elemental form tend tohave a higher activity for the formation of halide precursors. Elementsin the master alloy powders tend to provide a lower activity. Thus, forexample if it is desired to increase the diffusion of a given metal, itcan be added in elemental form to formulate the pack make-up.

The Argonne National Laboratory Publication ANL-5750 on “TheThermochemical Properties of the Oxides, Fluorides and Chlorides to2500° K” by Alvin Glassner and the publication “Thermodynamic Propertiesof Halides” by L. B. Pankratz, United States Department of the Interior,Bureau of Mines, Bulletin 674 provide useful information of relevance tofree energy of formation and thermochemical properties of halides, whichare of interest in AEMC coating formation. Table I (below) listsestimated free energy of formation values around 1340° K (about 1950°F.) coating temperature for some halides of elements of currentinterest. When dual activators such as chlorides and fluorides are used,it can be noticed from the listed values that the Yttrium and Hafniumchlorides exhibit energy values which are similar in magnitude to thoseof chromium fluorides. Thus with the aid of pure elements (with unitactivities) and master alloy make-ups (with lowered activities ofelemental metals) and use of single or multiple activators, that thehalides of chromium, hafnium, silicon and yttrium are formed. Theygenerate comparable vapor pressures for co-deposition of these metals onthe substrates through the various metal transfer mechanisms. TABLE IESTIMATED FREE ENERGY OF FORMATION Kcal/gram-atom of halide at 1340° KEstimated Value Halide (negative quantity) SiF₄ 82 HfF₄ 86 HfF₂ 90 HfF₃92 YF₃ 108 CrF₅ 56 CrF₄ 56 CrF₃ 65 CrF₂ 68 SiCl₄ 26 HfCl₄ 48 HfCl₃ 49HfCl₂ 52 YCl₃ 58 CrCl₄ 17 CrCl₃ 23 CrCl₂ 28

The binder is selected from those adhesives used in the art for bindingbraze tapes to metal surfaces. Such binders are commercially availablefrom welding material suppliers such as Wall Colmonoy Corporation andVitta Corporation. The binders may be glycerol based, petroleum based,and organic polymeric systems such as acrylic based, alginate based, andgelatin based binders. Typically the commercially available bindersinclude directions for combining the binder with a metal powder andother materials to form a slurry or paste.

Preferred activators include halide sources such as sources of fluorine,chlorine, and bromine. Acceptable activators include ammonium chloride,ammonium iodide, ammonium bromide, ammonium fluoride, ammoniumbifluoride, elemental iodine, elemental bromine, hydrogen bromide,aluminum chloride, aluminum fluoride, aluminum bromide, and aluminumiodide. Preferred activators include NH₄Cl and NH₄Fl. Also, it ispreferred to use dual activators, that is both a fluorine and a chlorinesource within the same tape, putty, and/or inert pack. Concentration ofthe halide source within pack 12 may be up to 20% by weight, and morepreferably is up to 8% by weight. In one preferred embodiment, thehalide concentration is between approximately 1% and approximately 3% byweight.

Inert materials include metal oxides such as alumina. Other preferredinert materials include kaolin, MgO, SiO₂, or Cr₂O₃. The inert fillersmay be used singly or in combination. Preferably the inert materialshave a non-sintered, flowable grain structure so as not to interferewith the gas transport diffusion of the desired metal.

In forming the slurry or putty form of the present invention, themetallic powder materials are combined with a binder. A filler materialmay also be included. An activator material may also be included. Themixture is formed to a desired viscosity or consistency. The slurry orputty may then be applied to a surface of a target item. The slurry orputty is typically applied to the surface by manual methods using a toolsuch as a spatula.

In a further embodiment, a tape is prepared using a chromium or chromiumalloy powder along with an active elements source master alloy and/orelemental powders. In forming a tape according to an embodiment of thepresent invention, a slurry or putty as described above is firstcreated. The slurry or putty preferably includes an inert fillermaterial such as aluminum oxide. In this embodiment a powder providingall desired metal sources is mixed with a binder. Known mixing andextrusion methods can be used to form tapes of a desired width andthickness. The Turbochrome division of Chromalloy is one source for themanufacture of tapes with a customer-specified composition.

If an activator is included in a tape, it is preferred that theactivator powder be in an encapsulated form. An encapsulated activatoris an activator, such as a halide compound, with a covering thatsurrounds the activator. The encapsulation thus acts to protect thehalide from the surrounding environment and also minimizes any reactionsthe halide compounds might otherwise undergo. The encapsulatingmaterial, typically an organic polymer, evaporates during heating atwhich time the halide compound is released to participate in thediffusion process. A practical advantage of using the encapsulated formof activator is that it extends the useful shelf life of a tape. Thustapes can be manufactured at one location and then distributed to repairfacilities. The tapes can then be stored at the repair facilities untilneeded without losing their effectiveness.

The tapes of one embodiment may also include an adhesive material.Preferably the adhesive material is added to a finished tape on anexterior surface of the tape. When a tape includes an adhesive, theadhesive can be used to secure the tape to a surface of a target item.In practice the adhesive vaporizes during heating and is removed; henceit does not significantly interfere with the diffusion process.Acceptable adhesives include pressure sensitive adhesives. Also, as isknown in the art, the adhesive layer may be covered with a backing orprotective wrap to prevent the tacky adhesive from bonding to surfacesprior to usage.

Optionally, the tapes may include structural or strengthening elementssuch as a ceramic gauze. Other metal oxides with binder may also providethe structural function of a ceramic gauze. The structural element, ifany, allows gases to flow therethrough so as not to interfere with gasphase diffusion.

The slurries, tapes, and putties of the present invention can havevarying concentrations of the metallic components within them. In oneembodiment, the chromium concentration is between about 10% to about80%; and the master alloy powder concentration of any one alloy Athrough E is about 1% to about 20% in weight. In another embodiment thechromium content is between about 10% to about 80%, silicon is betweenabout 0.5% to about 10%; hafnium is between about 0.5 to about 10%;yttrium is between about 0.5% to about 10.0%. In both these embodiments,there may be other elements added between about 1.0% to about 5%, wherethe other elements include refractory elements such as tantalum,rhenium, zirconium etc. Also to be included are alloys of these metals.These percentages are measured on a weight percentage basis comparingthe metal to metal concentrations. As a whole, the total metal componentin the slurries, tapes, and putties can be between about 20% to about90% with a range of 40% to about 90% being preferred.

The mixing steps described above for making tapes, slurries, and puttiescan take place using various kinds of equipment. For small batches, themixing step using equipment such as a blender found in an industriallaboratory is sufficient. For larger batches industrial-sized blendersand rollers may be preferred. U.S. Pat. No. 5,997,604 describesequipment and methods useful for forming tapes and is incorporatedherein.

Having described the invention from a structural and compositionalstandpoint, a method of using the invention is now described.

The tapes, slurries, and putties of the present invention are intendedfor use with known pack cementation methods. Referring now to FIG. 1there is shown an illustration of pack cementation equipment for usewith the present invention. A retort or box 10 provides a closedcontainer in which the target item rests. Box 10 may include a lid orother opening. If desired the lid may be affixed to the box structure asby welding so as to preclude the entrance of oxygen. Target 11 is placedwithin box 10. Box 10 and lid are composed of materials such asnickel-based superalloys or stainless steel metal capable ofwithstanding and suitable for heating to elevated temperatures underdiffusion coating procedures.

A pack 12 is also placed within box 10 such that pack 12 surroundstarget 11. Pack 12 includes inert materials. Additionally, pack 12 mayinclude activator materials.

The target item that is to be heat treated may receive a surfacepreparation in order to facilitate the diffusion process. Thepreparation may include an inspection, degreasing, and blast cleaning.Further the part may be rinsed with an evaporative solvent to remove anyremaining contamination residue.

A prepared slurry, putty, or tape may then be applied to a desiredsurface of a target item. The slurries and putties can be applied to adesired surface in a layer with a desired thickness. The tape embodimentis itself flexible so as to permit being shaped to cover complexsurfaces. Thus turbine blade air foils and vanes can be covered bybending and flexing a tape to cover a desired surface. If needed, anadhesive layer can be used to secure the tape to a given surface. Inpractice the tapes, without added adhesive, may display sufficienttackiness that the tapes will stick to a target item for sufficient timeto allow the item to be packed in a heating box. Surrounding the target,that has a tape on its surface, with the pack material, then serves tohold the tape, slurry, or putty in place. With respect to turbineblades, it is preferred to place the chromium with active elements tapeon the required portion of the turbine blade (such as the airfoil),while leaving the serration portion of the blade free of tape coverage.

If an activator is included in tape it is not necessary to includeactivator materials in the packing inside box 10. However, in analternative embodiment, dual activators are used in which one activatoris used in tape and a second activator, either the same or differentfrom the first activator, is included in the pack 12. If no activator isincluded in tape it is necessary to include an activator in the pack.

Once the materials for the pack 12 have been selected and assembled, andthe target item 11 has been taped in a desired location, the materialsmay be placed in box 10 and the box sealed. A heat treatment then takesplace. The coating development heat treatment includes heating the boxand contents at a constant temperature, up to 2100° F. for up to 16hours. A preferred heat treatment is heating at 1800° F. to 2000° F. for6 to 16 hours.

During the heat treatment mass transportation and diffusion processestakes place. Metal ions in the patch make-up react with halide ions.These molecules migrate to the surface of the target through gastransport process. At the surface of the metallic target the elementsneeded to form the active elements modified chromium coating diffusewith the materials in the target substrate. Temperature and time affectthe kinetics of this process. It is also preferred to carry out the heattreatment under an inert atmosphere or vacuum. In some embodiments argonor hydrogen can be flowed through the box in order to maintain an inertatmosphere and to assist with mass transport mechanisms.

In the following experimental work with only chromium containing tape isprovided, to illustrate the overall intent of AEMC coatings. Theformation of patch coating using the tape application as an exemplarymethod is shown in FIG. 2. In the illustrated specimen, coating waseffected only on the surface area showing bright contrast and the darkcontrasting area did not exhibit chromium addition to the surface. Thelocalized patch coating on the Mar M 247 cylindrical sample wasaccomplished by: applying three layers of 0.05 inch thick tape(designated as T-1558, obtained through Chromalloy Israel Ltd. asencapsulated ammonium fluoride activated tape) over partial length ofrod specimen; packing in a retort containing 2% ammonium chlorideactivator and 98% of 60 mesh aluminum oxide pack; heating the retort invacuum furnace with a partial pressure of protective argon gasatmosphere; and with a coating thermal cycle of 1975OF temperature for7.25 hours. The accomplishment of localized coating with tapeapplication is clearly illustrated in FIG. 2.

Also the formation of localized patch coating on the convex surface ofan airfoil is further represented in FIG. 3. With reference to thisFigure, the designation T-1553, T-1555 and T-1558 correspondrespectively to 30%, 50% and 80% chromium contained one layerapplication tapes.

Furthermore, the microstructures presented in FIG. 4 show the attainmentof comparably same coating thickness of about 0.5 mils for the twoconditions of three layers of 50% Cr and one layer of 80% Cr under thesame coating cycle. From the EDX analysis presented in FIGS. 5 and 6, itis apparent that the three layer (50% Cr tape) application resulted in ahigher concentration of 17.3% Cr when compared to one layer (80% Crtape) application which showed about 14% Cr in the patch coating.However, for the same coating cycle the three layer 80% Cr containingtape (T-1558/3 shown in FIG. 2) produced 22.4% Cr in the patch coating.Moreover, it should be apparent to those familiar with the art thatincreased patch coating thickness of 1 mil or over can be accomplishedby increasing the coating temperature from 1975°60 F. to approximatelythe 2050° F to 2100° F. range. Also, the chromium concentration in thepatch coating can be increased from 22.4% to approximately 30% byincreasing the concentrations of activators. Thus through theutilization of various degrees of metal powder concentrations, differenttypes and concentrations of activators, as well as application ofdifferent coating thermal cycles, it is taught per this invention, thedevelopment of AEMC coatings.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt to a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A method for diffusion coating a surface of a metallic targetcomprising the steps of: applying a tape to a surface of a targetwherein the tape is impregnated with metal powder comprising chromiumand active elements containing master alloy powder with elementsselected from the group consisting of silicon, hafnium, tantalum,rhenium, and yttrium; and wherein the tape further comprises a bindermaterial; placing a halide activator in proximity to the surface of thetarget; and heating the target to a temperature sufficient to causediffusion of the metals from the tape into the surface of the target. 2.The method according to claim 1 wherein the step of applying a tapefurther comprises applying a tape wherein the tape is impregnated with ahalide activator.
 3. The method according to claim 1 wherein the step ofapplying a tape further comprises a tape impregnated with metal powdercomprising a chromium alloy and at least two elements selected from thegroup consisting of silicon, hafnium, tantalum, rhenium, and yttrium. 4.The method according to claim 1 further comprising shaping the tape tocover a desired area of the target.
 5. The method according to claim 1wherein the step of applying a tape further comprises tape comprisinginert filler.
 6. The method according to claim 1 wherein the step ofheating the target further comprises heating the tape covered target toa temperature between approximately 1800° F. and 2100° F. and holdingthe temperature therebetween for between about 2 to about 16 hours. 7.The method according to claim 1 wherein the step of placing a halideactivator further comprises placing an encapsulated halide activator. 8.A method for diffusion coating a surface of a metallic target comprisingthe steps of: providing a metal powder comprising chromium and at leasttwo elements selected from the group consisting of silicon, hafnium,tantalum, rhenium, and yttrium wherein the metal powder has a mesh sizeof 140 or smaller; mixing the metal powder with a binder material;mixing a halide activator with the metal powder and binder material;mixing an inert filler with the metal powder, binder material, andhalide activator to form a tape; covering a desired surface area of thetarget with the tape; heating the tape covered target to a temperaturebetween approximately 1800° F. and 2100° F. and holding the temperaturetherebetween for between about 2 to about 16 hours thereby causingdiffusion of the metals into the target surface.
 9. A method fordiffusion coating a surface of a metallic target comprising the stepsof: providing a metal powder comprising chromium and at least twoelements selected from the group consisting of silicon, hafnium,tantalum, rhenium, and yttrium wherein the metal powder has a mesh sizeof 130 or smaller; mixing the metal powder with a binder material;mixing an inert filler with the metal powder, and binder material toform a tape; covering a desired surface area of the target with thetape; placing the tape-covered target in a diffusion box; packing amixture of halide activator and inert material in the diffusion boxaround the target; heating the slurry covered target in the diffusionbox to a temperature between approximately 1800° F. and 2100° F. andholding the temperature therebetween for between about 2 to about 16hours thereby causing diffusion of the metals into the surface of thetarget.
 10. A method for preparing a slurry for use in diffusion coatinga surface of a metallic target comprising the steps of: providing ametal powder comprising chromium and at least two elements selected fromthe group consisting of silicon, hafnium, tantalum, rhenium, and yttriumwherein the metal powder has a mesh size of 140 or smaller; mixing themetal powder with a binder material; mixing a halide activator with themetal powder and binder material; mixing an inert filler with the metalpowder, binder material, and halide activator to form a slurry; applyingthe slurry to a surface of a target in a desired thickness.
 11. Themethod according to claim 10 wherein the step of providing a metalpowder further comprises a metal powder comprising a chromium alloy andat least two elements selected from the group consisting of silicon,hafnium, tantalum, rhenium, and yttrium.
 12. The method according toclaim 10 further comprising covering a desired area of the target withthe slurry.
 13. The method according to claim 10 wherein the step ofmixing a halide activator further comprises mixing an encapsulatedhalide activator.
 14. A method for diffusion coating a surface of ametallic target comprising the steps of: providing a metal powdercomprising chromium and at least two elements selected from the groupconsisting of silicon, hafnium, tantalum, rhenium, and yttrium whereinthe metal powder has a mesh size of 140 or smaller; mixing the metalpowder with a binder material; mixing a halide activator with the metalpowder and binder material; mixing an inert filler with the metalpowder, binder material, and halide activator to form a slurry; applyingthe slurry to a surface of a target in a desired thickness; covering adesired area of the target with the slurry; heating the slurry coveredtarget to a temperature between approximately 1800° F. and 2100° F. andholding the temperature therebetween for between about 2 to about 16hours.
 15. The method according to claim 14 wherein the step ofproviding a metal powder further comprises a metal powder comprising achromium alloy and at least two elements selected from the groupconsisting of silicon, hafnium, tantalum, rhenium, and yttrium.
 16. Themethod according to claim 14 wherein the step of mixing a halideactivator further comprises mixing an encapsulated halide activator. 17.A method for diffusion coating a surface of a metallic target comprisingthe steps of: providing a metal powder comprising chromium and at leasttwo elements selected from the group consisting of silicon, hafnium,tantalum, rhenium, and yttrium wherein the metal powder has a mesh sizeof 140 or smaller; mixing the metal powder with a binder material;mixing an inert filler with the metal powder, and binder material toform a slurry; applying the slurry to a surface of a target in a desiredthickness; covering a desired area of the target with the slurry;placing the slurry-covered target in a diffusion box; packing a mixtureof halide activator and inert material in the diffusion box around thetarget; heating the slurry covered target in the diffusion box to atemperature between approximately 1800° F. and 2100° F. and holding thetemperature therebetween for between about 2 to about 16 hours.
 18. Aturbine blade ready for diffusion comprising: a turbine blade defining asurface; a tape affixed to a surface of said turbine blade wherein saidtape comprises a binder, an inert filler, a halide activator, a chromiumcontaining powder, and a powder containing at least two metals selectedfrom the group consisting of: silicon, hafnium, tantalum, rhenium, andyttrium.
 19. The turbine blade according to claim 18 wherein said tapefurther comprises an adhesive.
 20. The turbine blade according to claim18 wherein said tape is affixed to the airfoil of the turbine blade.